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Research on Pattern Formation in 1D Dielectric-Barrier Discharges |
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| Dr. Matthew Walhout
Summer Students 1998 Jon Niehof Senior Research Projects Funding from Calvin College, Research Corporation, and |
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| What is a dielectric-barrier discharge?
Dielectric-barrier discharges (DBD’s) comprise a specific class of high-voltage, ac, gaseous discharges that typically operate in the near-atmospheric pressure range. Their defining feature is the presence of dielectric layers that make it impossible for charges generated in the gas to reach the conducting electrode surfaces. With each half-cycle of the driving oscillation, the voltage applied across the gas exceeds that required for breakdown, and the formation of narrow discharge filaments initiates the conduction of electrons toward the more positive electrode. As charge accumulates on the dielectric layer(s) at the end(s) of each filament, the voltage drop across the filament is reduced until it falls below the discharge-sustaining level, whereupon the discharge is extinguished. The low charge mobility on the dielectric not only contributes to this self-arresting of filaments but also limits the lateral region over which the gap voltage is diminished, thereby allowing parallel filaments to form in close proximity to one another. Thus, the entire gas-filled space between parallel electrodes can become, on average, uniformly covered by transient discharge filaments, each roughly 0.1 mm in diameter and lasting only about 10 ns. The DBD’s unique combination
of non-equilibrium and quasi-continuous behavior has provided the basis
for a broad range of applications and fundamental studies. Its
use in industrial ozone reactors has generated interest in optimizing
conditions for specific chemical reactions. To this end,
experimental DBD studies have explored different gas mixtures,
electrical characteristics, and geometries [1-3]. Related work
has focused on maximizing the ultraviolet radiation from excimer
molecules produced in DBD’s [4-6]. Several groups have modeled
single-filament dynamics in order to account for the many two- and
three-body reactions involving electrons, ions, neutral atoms, and
photons [7-10]. These efforts have been moderately successful in
explaining and predicting the chemical and radiative properties of
various DBD systems. On another research front, it has been seen
that the transverse spatial distribution of discharge filaments in 2D,
parallel-plate DBD’s can take the form of stable, large-scale patterns
reminiscent of those associated with magnetic domains or
Rayleigh-Bénard convection [11-14]. These patterns have been
modeled with some success using methods that apply generally to pattern
formation in nonlinear dynamical systems [15-17]. Thus, the
dynamical interactions between filaments, as well as the chemical and
electronic interactions within filaments, have proven interesting. Experimental Setup Our
discharge cell is a 30-cm-long, cylindrical glass tube with inner
diameter 2.0 mm and outer diameter 7.5 mm. Stripes of silver
paint, 3 mm in width, are applied on diametrically opposite portions of
the outer surface. In order to prevent sparking around the
outside of the cell, we enclose the discharge tube and electrodes in a
larger glass tube and fill the annular space with insulating
transformer oil. The electrodes extend through rubber seals at
opposite ends of the tube, providing the contacts across which a
sinusoidally varying high voltage is applied. The 1-kHz-to-20-kHz
signal is derived from a function generator, fed through a 2 kW audio
amplifier, and then stepped up with a high-voltage transformer.
Adjustable balance valves on either side of the tube provide precise
control over the rate and direction of mixed gas flow through the
discharge region. SCHEMATIC DIAGRAM: PHOTO OF SETUP: Owing to the curvature of the inner surface of the tube, our DBD is tightly constrained in one lateral dimension and supports purely 1D patterns along the tube’s length. Having chosen a tube with an inner radius smaller than the characteristic size of a discharge footprint (ie. patch of deposited surface charge), we observe a suppression of any filaments that are not localized to the electrode axis. Thus, we call this an effectively 1D system. We use two diagnostic tools in the present experiments. First, a camera and computerized image- acquisition system allow us to record pictures of the discharge and to analyze the digitized frames. Second, the circuit shown below allows us to monitor the voltage across the tube (V = Vosin(2pi*ft)) and the voltage across a series capacitor (Vc), the latter being proportional to the charge (Q) stored on the electrodes. In the voltage range below breakdown, a Vc-vs-V oscilloscope trace (or QV plot [10]) falls along a line with slope proportional to the capacitance of the discharge tube. With a discharge running, the jumps in Q corresponding to distinct discharge stages cause the trace to take on an open, sliver-like shape. As will be seen below, these jumps help us understand the DBD pattern-formation dynamics.
Results With
a 2:1 mix of He and Ar, interesting spatial structures appear as Vo is
increased. The images below were taken with a color digital
camera. Stationary (repeating) filaments are seen as bright,
vertical stripes that widen at the tube wall. The patterns formed
by these stationary filaments are of three distinct kinds, which we
give the following names: Type A patterns appearing in the
general range of 450 V < Vo < 610 V and exhibiting narrow
filaments with relatively wide separations;Type B patterns (610 V <
Vo < 940 V), with narrow filaments separated by roughly half of the
typical Type A spacing; and Type C patterns (980 V < Vo < 1200
V), characterized by alternating bright, wide filaments and dim, narrow
filaments. Type A (450 V < Vo < 610 V) Type B (610 V < Vo < 940 V) Type C (980 V < Vo < 1200 V) Spatial disorder (Vo > 1200 V) In
the voltage range between 940 V and 980 V, individual filaments strike
randomly at apparently uncorrelated positions. This type of
behavior, also seen for Vo > 1250 V, is suggestive of spatiotemporal
chaos. Such a characterization must be qualified by the
time-resolved Vc data shown below. In plots a-c, sharp steps in
Vc reveal that discharge types A, B and C are associated, respectively,
with the presence of 1, 2 and 3 distinct discharge stages in each
voltage half-cycle. However, for conditions producing spatial
disorder on time scales down to the half-cycle, all but one or two of
the Vc steps disappear and discharges occur over an extended interval
during each half-cycle (plot d). The disappearance of Vc steps
indicates that an abrupt increase in temporal disorder accompanies the
transition to spatial disorder. Top: Replica of the driving voltage signal. After measuring the average filament spacing (lambda) for a wide range of Vo settings, we obtained the following graph: Discussion To begin to understand the observed patterns, we would like to answer the following questions: 1. Why do filaments remain stationary? We're actually still working on all of these questions, we have some answers to 1 and 2:
Explanation for transition from Type A pattern to Type B pattern The following sequence of pictures shows what we think happens in the system References [1] B. Eliasson and U. Kogelschatz, IEEE Trans. Plasma Sci. 19, 309 (1991). |
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